Conventional schemes for dosing a discharge lamp with mercury involve directly adding liquid mercury to the arc tube or lamp through an exhaust tube having a narrow diameter. Problems with such mercury dosing schemes occur in the separation and transport of small amounts of mercury into the lamp. Often, for example, droplets of mercury are left in the manufacturing equipment and the exhaust tube. Hence, in order to avoid having an insufficient quantity of mercury in the discharge lamp, the lamp is typically dosed with an excess of mercury. In fluorescent lamps, for example, there is no performance penalty for adding too much mercury because excess mercury resides on the inner surface of the lamp as condensate. An exemplary fluorescent lamp may be dosed with 50 to 100 mg of mercury, even though only 5 to 10 mg are needed for operation over a 20,000 hour life, for example. However, for any type of lamp, there are environmental and cost considerations in using too much mercury. Moreover, for high-pressure mercury and metal halide arc tubes, wherein the entire mercury dose is commonly vaporized during lamp operation, the addition of too much mercury results in an operating voltage that is too high.
Hence, it is desirable to provide a more precise method of dosing discharge lamps with mercury. Unfortunately, greater precision typically results in a slower manufacturing speed and a more complex manufacturing process. For example, mercury drops in the range from 1 to 10 mg have volumes in the range from 0.07 to 0.7 cubic mm, respectively. Such small volumes, in addition to surface tension effects, make measuring and handling the mercury drops difficult.
As described in Holmes U.S. Pat. No. 4,754,193, one method of mercury dosing involves capturing a controlled amount of liquid mercury in a container or a porous carrier prior to lamp or arc tube assembly. The mercury container or carrier is then employed as an internal component of the lamp or arc tube. Although such a method does not require direct measurement or handling of liquid mercury during lamp assembly, it is still complex and costly.
Accordingly, it is desirable to provide a new and improved, high-precision, high-speed method for dosing a discharge lamp with mercury which does not involve direct measurement or handling of liquid mercury.